Refining of metals



June 28; 1938. F. HANSGIRG REFINING 0F METALS 2 Shets-Sheet 1 Original FiLed Dec. 21, 1934 COOL/N6 FLU/D HYDROGEN VACUUM INVENTOIZH km. 47%, L3 MW HYDROGEN June 28, 19538 A g m 2,122,139

{ REFINING 0F METALS Original Filed Dec. 21, 1934 v 2 Sheets-Sheet 2 HYDROGEN HYDROGEN Patented June 28, 1938 F TAILS Fritz Hansgirg, Konan, Korea, assignor to American Magnesium Metals Corporation, Pittsburgh, Pa, a corporation of Delaware 4 Claims.

This application is a division of my prior application Serial No. 758,622, filed December 21, 1934.

This invention relates to the distillation or sub- 5 limation of impure metals, particularly for refining metals in finely divided form.

The invention is applicable generally to the treatment of metals and materials containing metals susceptible of evaporation, e. g. distilla- 20 tion, especially those produced in finely divided form, such as magnesium and zinc. poses, of illustration, however, it will be described with particular reference to the treatment of magnesium, to which it is especially suited.

It has been proposed heretofore to refine such metal-containing materials by converting them to granular form and heating the granular material, to eiiect distillation or sublimation, by passing an electric current therethrough. Such 20 procedures have not become commercially practical because of disadvantages that have inhered in them as previously proposed. The granulation may be accomplished, according to prior suggestions, by mixing the powdered metal-containing 25 material with conductors and semi-conductors of the electric current, which in the case of magnesium might be carbon and magnesium oxide, and adding to the mixture a bonding agent, suitably a carbonizing binder, such as hydrocarbon 30 oils. The mixture is subjected to heat to cause evaporation of hydrocarbon oil and to form a granular solid mass in which the pulverulent materials are bonded by a coked residue which is formed in volatilization of the hydrocarbon oils.

Although by using such granular material as a current conductor it is possible to develop the lid amount of heat necessary'for vaporization of the magnesium to accomplish distillation or sublimation, this has not been commercially realizable so-previously. One reason is that such materials have the disadvantage that their conductivity changes materially with the temperature, that is to say, that the conductivity decreases with increasing temperature. Also, as the magnesium, 4.5 or other metal such as zinc, which is contained in thematerial is distilled away, the conductivity decreases further. This in combination with the types of furnace suggested previouslyhas acted detrimentally.

For instance, if sucha material is subjected to heating by current conduction in a shaft furnace of circular cross section, such as that recommended by Nathusius for the reduction of zinc ore, the use of segmental electrodes is unavoidable 55and this entails variations incurrent conduction For pur- (Cl. l510) in consequence of the difierences in distance between the curved, segmental electrodes. This causes the granular material, which moves downwardly in the shaft, to be heated to varying degrees. Through decrease of the resistance the hotter spots are then favored, which causes further stronger heating, so that undesirable overheating occurs in the charge at a few spots. Care must be taken in the evaporation of magnesium if a pure product is to beobtained, and such variations in heating lead to disturbances in the process and through-local melting phenomena may even cause complete interference with operation.

According to the recommendations of Nathusius six segmental electrodes are positioned in three horizontal planes in the shaft, the electrodes being connected to the six poles of a non-coupled threephase current system. Through this it should follow that the current not only flows horizontally across the direction of movement of the granular material, but also diagonally, and so lead to the greatest possible uniformity of heating. But this arrangement can not overcome the difliculties pictured hereinabove.

In the previously proposed zinc distillation apparatus the vapors pass from the shaft-like distillation chamber to the outside and are condensed in a separate condenser. Such a procedure is not practicable for the distillation of magnesium, particularly under vacuum, because the magnesium in consequence of its low specific gravity per unit of volume possesses an intrinsically small heat of vaporization, which is liberated as heat of condensation in the condensation step. Through working in vacuum the resultant dilution causes the heat of condensation per unit volume of magnesium vapor to be significantly smaller, amounting, for example, in the case of distillation under'a vacuum of 40 millimetres-to I only about $3 of the heat of condensation per unit of volume of zinc vapor when distilled under atmospheric pressure.

In order toaccomplishtheiliquefaction of such a dilute metal vaporin a condenser with the greatest possible freedomfrom loss it is necessary to cool the vapors extremely slowly in a'large surfaced condenser. But since the vapors to be con.- densed' carry a relatively smallamount of heat per. unit of volume, the condenser-must be heated from the outside for condensation of thema gnesium vapors in vacuum, in order to obtain that temperature which is necessary to condense (the metal vapor to liquid .without loss. ,Thisprocedure requires a certain energy input in order to bring the condenser to the correct initial temperature.

It is among the objects of the invention to provide a method of distilling or subliming metal of the general type just referred to, in which the granular material containing the metalis heated by passage of an electric current, which provides for uniformly heating the distillation stool; through its cross section, which avoids or minimizes the foregoing and other disadvantages, is readily and easily controllable, provides operating economies, and permits commercial and economical application of the general principle involved.

A further object is to provide an apparatus for practising the method embodied in the invention, which is compact, relatively inexpensive, easily operated and controlled, and possesses the advantages which characterize the method.

Among still others, another and special object is to provide an apparatus in which the metal vapors are received and condensed in a condenser embodied in the distillation apparatus, whereby to avoid the foregoing named disadvantages in condensation, especially of magnesium vapor, and to afford maximum heat economy and yield of purified metal.

The invention will be described in connection with the accompanying drawings, which represent its preferred embodiment. Fig. 1 is a longitudinal vertical section through the furnace; Fig.

' 2 a view similar to Fig. 1 showing the upper part of the furnace to an enlarged scale; and Fig. 3 a cross section taken on line III-III, Fig. 1.

The invention is practised in connection with granular stock. If the material is not in such form it is granulated, advantageously by mixing it with carbon, with or without another conductor or semi-conductor, and a suitable binder. For example, powdered magnesium is mixed with granular carbon or granular magnesium oxide, or both, to provide bulking agents capable of conducting electric current, at least to someextent. To the mixture is added a binder, suitably a material which carbonizes when heated, of which the hydrocarbon oils constitute the preferred example. Enough oil is added to produce a paste. The mixture is heated to expel the excessoil and cause bonding. If heated en masse the coked product is broken up to form granules of suitable size.

In accordance with the invention the granular stock is passed through a distillation chamber of substantially rectangular cross section. For instance it is passed downwardly through a vertical shaft of square cross section in which it is heated. Electric current is passed through thestock in a direction transverse to the direction of movement of the stock. To this end the current flows through the stock in zones from one face of the shaft to the opposed face, and in accordancwith the invention this is accomplished by the use of flat, or plate-like electrodes disposed in opposed pairs at opposite surfaces of the shaft, the pairs being spaced longitudinally of the shaft. This avoids the disadvantages attendant upon the use of segmental electrodes, for with the plate electrodes of this invention the electrodes of an opposed pair are uniformly spaced at all points, thus providing a condition ideal for uniform flow of current through the column of material which moves through the shaft.

The electrodes are not connected vertically, and

being disposed in horizontal zones there are provided current conducting zones alternating with current-free zones. All of the electrode plates on each side are supplied with current at the same potential, the current being supplied to the individual pairs of plates which, as just noted, are not connected vertically. In the apparatus provided by the invention means are provided for passing current to or cutting it from the individual pairs of opposed electrode plates, according to need. The provision of conditions for uniform current flow in a given zone, and of means for cutting current into or from the separate zones makes it possible to control the heating and avoid irregularities thereof, since the flow in any zone is uniform and further heat in any region can be supplied or avoided by switching appropriate pairs of plates into or out of the circuit.

In order that the magnesium vapor liberated in the distillation need not be passed to a condenser situated outside of the shaft furnace, the invention provides for condensation of the vapor in the furnace structure. To this end the vapor is passed into a condenser which suitably divides the shaft into two vertical halves. Preferably this is accomplished by suspending from the top of the distillation shaft and centrally therein a tubular metallic member of substantially rectangular section. The section of this member is less than that of the shaft to provide a passage on each of two sides for the distillation stock, and it extends to a region of the furnace at which there is attained a temperature which lies above the boiling point of the metal. From the bottom of the shaft there projects upwardly within the lower end of the tubular member a similar tubular member but of lesser cross section, whereby the juncture of the two tubes provides a slot-like opening into the interior of the condenser. The distillation stock flows on opposite sides of the condensing chamber thus formed and at the juncture of the two tubes it forms a natural slope through which the metal vapor evolved from the granular material escapes into the inner condenser chamber.

In the centrally arranged condenser chamber one, two or more tubes, closed at their lower ends, project from above downwardly to a zone at which the temperature in the condensing chamber is above the condensation point of magnesium, or above that of the particular metal being distilled. These tubes are cooled from within by circulation of a cooling fluid to bring the surface of the cooling tubes to such a temperature that the metal, such as magnesium vapor is condensed to liquid thereon and drips oil at the lower end of thetubes. Most suitably the condenser tube wall indirectly cooled, as described hereinafter, for example.

The lower tubular member projects in gastight manner downwardly through the bottom of the shaft and its outer portion is filled with oil into which the magnesium drips, in the manner described in my application Serial No. 722,278, filed April 25, 1934, and in the manner there described is recovered in granular form.

Having reference now to the drawings, the apparatus showncomprises a distillation shaft I formed, for example, between refractory walls 2 enclosed within a mantle 2a. This shaft is of substantially rectangular section, as seen in Fig. 3,

movement of the stock between a plurality of till electrode plates 3 and 4 disposed in opposed pairs at two opposite faces of the shaft. ,The disposition of the pairs of electrode plates is such that the current zones alternate with zones in which no current is passed. To avoid current scattering the electrode plates may be somewhat shorter than the face of the shaft, as seen in Fig. 3. As shown in Figs. 1 and 2, the opposed pairs of electrode plates are spaced vertically from the immediately adjacent pairs by a distance not exceeding the height of the electrode plates.

, The vapor liberated from the distillation stock is condensed in a condensing chamber formed of rectangular tubular metallic members 5 and t disposed centrally in distillation shaft II. Memher 5 is suspended from a gas-tight shaft cover l and it extends down to a region in which the stock reaches a temperature above the boiling point of magnesium. As seen in Fig. 3, one dimension of member 5 is such that two of its faces contact two opposed faces of shaft 11, its other dimension being such that there are provided two channels 8 and 9 for the stock 3 to flow downwardly along opposite sides of the condenser structure. Member t projects upwardly through the bottom of shaft i into the lower end of member 5. Member 6 is wide enough to contact the inside of the walls of member 5 which lie against the shaft, but its depth is less than that of member 5, as seen in Fig. 3. Thus the opening in distillation channels 8 and 9 increases at the juncture of the two condenser elements, forming a slope in the distillation stock through which the metal vapors evolved from the stock escape through the 'slot-like openings it into the interior of the condenser.

Most suitably members 5 and t are provided 1 with inter-engaging longitudinal outstruck noses ii and i2, respectively, noses it being disposed in,

complementary grooves formed in shaft i. This construction fixes the position of the condenser members in the shaft and permits the two membersto move relative to one another and the shaft as they expand or contract longitudinally.

Mounted within the condensing chamber are a plurality of tubes it closed at their lower ends and cooled lnteriorly by a suitable cooling fluid, such as water, oil orair, circulated therethrough. Advantageously the construction is' such that the condenser tubes are cooled indirectly. In the embodiment shown the cooling fluid is introduced through a tube id mounted within another tube Ma, closed at its lower end, which is disposed coaxially within condenser tube it. The cooling fluid thus is introduced through tube it and is withdrawn from the top of tube Ma, whereby the transfer of heat from the wall of condenser tube it to the cooling'fluid circulated in tubes iii and lid takes place principally by radiation. The construction may be such that tubes (Hi and Ma are movable vertically in tube iii to vary the cooling effect in that manner. The cabling may be controlledalso by the character of cooling fluid used, its initial temperature, and the rate of flow through the condenser.

The tubes it are cooled to such an extent that magnesium vapor entering through slots it is condensed to liquid. The lower end of tubular member 6 projects, in gas-tight connection, through'the bottom of the furnace and contains a body of oil it into which the liquid magnesium drips from the ends of tubes it.

The system may be evacuated, if desired, through a pipe 88. Granular distillation stock is supplied continuously to channels t and a through pipes ii and it provided, respectively, with lock valves, or chambers, indicated schematically at l9 and 20, to maintain the system under vacuum where used, and to prevent escape of metal vapor.

Pyrometers 2i extend through the refractory walls 2 into channels 8 and 9, and pyrometers 22 project into the condensing chamber, for measuring the temperatures and controlling the distillation and condensation conditions. While the pyrometers are shown at certain levels in the distillation and condensing shafts, and also disposed at various positions laterally thereof (Fig. 3), it will be understood that more or fewer pyrometers may be used, and that their positions are more dependent on the conditions prevailing and the control desired. 7

The granular distillation residue which reaches the bottom of the shaft is removed by screw conveyors 23 and 2 5 whose outlets are suitably sealed to prevent entry of air into and escape of metal vapor from the apparatus. The granular solid pure magnesium which collects in oil i5 is removed at intervals through a lock chamber indicated schematically at 25.

To avoid escape of the magnesium vapor the cooler upper portion of the condensing chamber and the upper ends of tubes l3 and Ma are surrounded by heat insulated inserts 26 and 21!, of slag wool for example, which extend so far downwardly that a temperature zone is obtained which always lies above the condensation point of the metal.

Hydrogen is introduced from a pipe 28 into the space between the heat insulated insert it and the wall of tube 5 to avoid a penetration of the 'magnesiun vapors upwardly into this portion of the apparatus. Similarly the two feed conduits l9 and iii are provided with connections 29 for the introduction of hydrogen, to provide a flow of gas which drives any metal vapor escaping into the upper part of the shaft through the opening it into the inner condenser chamber. Hydrogen can likewise be introduced through screws 23 and 26 in order to drive the vapors evolved in the lower part of the shaft in the direction of opening iii, where they pass into the centrally arranged condensation chamber.

It will be understood that the entire furnace is of gas-tight construction to avoid loss of metalvapor or ingress of air.

In the practice of the invention using the apparatus just described granular stock is fed continuously to the top of the shaft, flows downwardly, is heated by current flowing across the column to a temperature such that the metal, magnesium in this case, is vaporized,the vapors pass into the condenser, which is regulated to convert them to liquid, and the residue from the distillation is carried away by the screw conveyors. For a definite velocity of flow of the granular material the temperature of the stock in the separate horizontal zones is so regulated by switching current to and from the individual opposed pairs of plates that the desired temperature in the zone concerned is reached. Through this procedure of distributing the electrical energy input in horifurnace connected by leads ill, each provided with a switch 34a, to a common bus 3| connected to one pole of a suitable generator, either A. C. or D. C. On the other side of the furnace current is supplied by a bus 32 connected to the other pole of the generator, but on this side each of the conductor plates 3 is connected to the bus 32 by a lead 33 containing a switch 34b, so that by operation of switches 3M and 3511 current may be supplied to or cut off from any of the individual pairs of opposed plates according to need. The switches may be operated by hand upon observation of the readings of the pyrometers, or they may be operated automatically from pyrometers 2| by means, such as relays, known in the art.

The two channels formed on opposite sides of the condenser chamber being filled with granular distillation stock, the current flows from the electrode plates through the granular material in one channel, e. g. 8, to the centrally disposed metallic condenser elements 5 and 6, through the metal walls thereof to the other side and from here through the granular material in the other channel, 1. e. 9, to the electrode plates on the opposite side of the shaft. By this means the centrally arranged condenser, into which the magnesium vapor escapes, serves simultaneously as a central electrode and also upon two sides is surrounded by hot granular material heated tothe distillation temperature, so that there is obtained a supplementary heating for the centrally disposed condenser space.

Through the invention itis possible to provide a shaft furnace with a double shaft and a centrally disposed condensation chamber, to apply as a distillation apparatus for granular material containing magnesium or other metal, whereby through the process of the zone-like distribution of the current conduction and the periodic operation of the current introduction to the various zones the granular material containing magnesium is heated exactly to the temperatures which are necessary to evaporate the magnesium without the possibility of overheating of the granular material. This flexibility in temperature control is of great importance, particularly in connection with magnesium powder which has been produced by electrothermic reduction of magnesium oxide with carbon, for the preparation of granular material which is to serve for distillation, to avoid overheating the material in the distillation.

The magnesium powder prepared according to my Patent No, 1,884,993 commonly contains, in consequence of the reoxidation which cannot be wholly avoided, a certain proportion of carbon and magnesium oxide, but for the provision of a solid granular residue after distillation there is added further carbon and magnesia.

Likewise it contains in consequence of the high reduction temperature which prevails, also other constituents of magnesite, such as iron, silicon, calcimn and aluminum in the form of free metals or their carbides. If such material is heated over the boiling point of magnesium under the conditions of vacuum prevailing, the carbon content of the charge may cause a reduction process to occur which causes formation of carbon monoxide, which latter in the condensation prevents the separation of liquid magnesium. Also the accompanying impurities, such as calcium and silicon, can simultaneously evaporate and contamimate the condensed magnesium and render it less useful. It is'therefore of outstanding significance that the distillation process of this invention can be so carried on that in the heating of the granular material at no-point is a temperature such as to produce these effects, say 950 C., reached or essentially exceeded. In the use of a corresponding vacuum this temperaturecan be essentially lower.

The arrangement of the central condensation chamber permits the magnesium vapor to be conducted the shortest distance'for condensation and the hot granular charge of material to be'distilled serves simultaneously for heating the centrallyarranged condensation shaft.

Since the resistance of the granular material used for distillation decreases with increasing temperature, while on the other hand, however, the use of the same voltage in all of the heating zones possesses a certain advantage, the application of a two-step distillation shaft is advantageous since in the upper part of the shaft the current path is shorter than in the lower part, where the resistance of the material is lowered.

A regulation of the temperature relations can also be introduced by variation of the width of the conductor strips in the separate heating zones, as also by the separation of the zones free from current which alternate with the current-carrying zones.

A further feature of importance which accrues from the invention is the provision of an apparatus in which the unbalanced pressure forces created in the use of vacuum are applied to the cold, or relatively cold, outer portion of the furnace, such as its mantle, instead of to heated metallic parts as has been the case with prior distillation apparatus. Thus, the condenser is mounted within the distillation chamber, so that for all practical purposes both sides of the condenser are exposed to substantially the same pressure. The mantle, top and bottom members, and all connections and other elements are so associated as to form a gas-tight casing enclosing the furnace shaft and other parts, wherefore the casing receives the stresses due to the vacuum in the furnace. But in the construction provided by this invention the shaft mantle is not highly heated, so that the mantle and its associated parts are fully able to withstand the stresses cre ated by the vacuum in the furnace, in contrast with furnaces in which such unbalanced forces must be taken up by more or less highly heated metallic elements. Thus the refractory masonry serves to form the rectangular distillation shaft and also to insulate the charge in the shaft against loss of heat by radiation while keeping the outer gas-tight casing cool to resist the unbalanced forces set up by the vacuum within the furnace.

According to the provisions of the Patent Statutes, I have explained the principle and operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, Within the scope of the appended claims, the invention may be practised otherwise than as specifically illustrated and described.

I claim:

1. The method of recovering compact metallic magnesium from powdered magnesium comprising mixing said magnesium with an electrical conducting material, such as carbon and magnesium oxide, and a binder, converting the mixture to granular form, passing the granular material downwardly through a pair of parallel vertical shafts of substantially rectangular cross section separated bya metallic condenser, passing electric into and from the separate zones, passing the magnesium vapor formed into said condenser and condensing it to liquid form.

2. The method of recovering compact metallic- "magnesium from powdered magnesium comprising mixing said magnesium with an electrical conducting material, such as carbon and magneslum oxide, and a binder, converting the mixture to granular form, passing the granular material downwardly through a vertical shaft of substantially rectangular cross section, passing electric current through the material in separated zones transverse to the direction of its movement between electrodes disposed in opposed pairs at two opposed sides of said shaft, regulating the temperature of the material by switching current into and from the separate zones, passing the vapor into a metallic walled condensing chamber of substantially rectangular section disposed centrally within and extending longitudinally through said shaft, condensing the vapor on a cooled surface in said condensing chamber to cause liquefied magnesium to collect thereon and drip therefrom, and collecting the liquid magnesium in a body of "oil disposed below said cooled surface in said condensing chamber.

3. In a method of recovering magnesium in compact form from material containing metallic magnesium, the steps comprising passing said material downwardly through a vertical shaft of a rectangular cross section of substantial uniform area, conveying through the material electric current in horizontally separated streams between a plurality of electrodes disposed in hori- Zontally opposed pairs at opposite vertical faces of said shaft, and regulating the temperature of the material by switching current into and from the horizontally opposed pairs of electrodes.

4. In a method of recovering magnesium in compact form from material containing metallic magnesium, the steps comprising passing said material downwardly through a vertical shaft of a rectangular cross section of substantial uniform area, conveying through the material electric current in horizontally separated a plurality of electrodes disposed in horizontally opposed pairs at opposite vertical faces of said shaft, and separated vertically by a exceeding the electrode height, and regulating the temperature of the material along substantially the entire shaft by switching current into and from said horizontally opposed pairs of electrodes FRITZ HAN SGIRG.

streams betweendistance not 

